QuickLinks -- Click here to rapidly navigate through this document

FORM 6-K
UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

Report of Foreign Issuer

Pursuant to Rule 13a-16 or 15d-16
of the Securities Exchange Act of 1934

For the month of May 2003   Commission File No.: 0-29954

THE THOMSON CORPORATION


(Translation of registrant's name into English)

Metro Center, One Station Place
Stamford, Connecticut 06902, United States
(Address of principal executive offices)


Indicate by check mark whether the registrant files or will file annual reports under cover Form 20-F or Form 40-F.

Form 20-F o
 
Form 40-F ý

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1): o

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7): o

Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

Yes o
 
No ý

If "Yes" is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82-              


EXHIBIT INDEX

Exhibit Number
   Description
99.1   Management's Discussion and Analysis
99.2   Unaudited Consolidated Financial Statements for the periods ended March 31, 2003 and 2002 and as at March 31, 2003 and December 31, 2002 (including reconciliation to U.S. GAAP for the periods ended March 31, 2003 and 2002)
99.3   Controls and Procedures
99.4   Certifications Pursuant to Section 302 of the Sarbanes-Oxley Act of 2002
99.5   Certifications Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002

2


SIGNATURES

        Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

 
    
    
        THE THOMSON CORPORATION

 
 
 
 
 
    By:   /s/ Paula R. Monaghan
Name: Paula R. Monaghan
Title: Assistant Secretary
Date: May 6, 2003        

3



QuickLinks

EXHIBIT INDEX
SIGNATURES